Carlo Bartoli

Validation of the DJOSER analytical thermal simulator for electronic power devices and assembling structures

The present communication deals with the tests for the validation of the DJOSER steady-state thermal simulation program, purposely designed for power electronic assembling structures and which is based on the resolution of analytical relationships. The validation experiments were carried out theoretically by comparing the thermal maps with those obtained using standard finite-elements programs and yielding temperature accuracy below 1%. Experimental tests were also performed on purposely built multi-layer structures and industrial circuits with power diodes mounted in naked-chip configuration. The simulated maps were compared with accurate thermo-graphic recordings and showed a good agreement, testifying the validity of the mathematical model.

Impingement heat transfer at a circular cylinder due to a submerged slot jet of water

Abstract An experimental investigation was carried out on the heat transfer due to a submerged slot jet of water impinging on a circular cylinder in crossflow. The cylinder diameter and the slot width are of the same order of magnitude, specifically D s = 2.0 and 3.0 mm and D c = 2.5 and 3.0 mm . The experimental apparatus allowed variation of the slot width, the cylinder diameter, and the distance from nozxle exit to heater. Conditions of impingement from the bottom (ascending flow) were taken into consideration as well as impingement from above (descending flow). The Nusselt number was determined as a function of Reynolds and Prandtl numbers in the range 1.5 × 10 3 4 , 2.7 1.5 ≤ z/D s ≤ 10 . The experimental data were correlated with a simple equation that fits 90% of the data with a precision of 20%.

Forced and mixed convection heat transfer from an array of cylinders to a liquid submerged jet

Abstract The results of an experimental investigation concerning the heat transfer from three cylindrical heaters to a water jet are reported in the form of correlating equations, which express the Nusselt number versus the Reynolds, Prandtl and Grashof numbers and some dimensionless ratios characterising the configuration. As the experienced range of the thermal flux is wide (2·104 ≤ q ≤ 6·105 W·m−2), the influence of the free convection, which was shown to be negligible in previous studies, is carefully investigated in the present one. This influence appears still negligible up to the maximum value of q for the heater impinged by the jet; on the contrary it is remarkable for the heaters lying in its wake. Another aspect which is carefully studied is the influence both of the ratios characterising the configuration and of the impingement direction: accordingly the values of these ratios and the kind of impingement which yield the maximum Nusselt number are clearly singled out. The investigation is completed by some visualization experiments which allow us to qualitatively clarify some aspects of the interaction between the dynamic and thermal fields.

Heat transfer and flow pattern at a cylinder impinged by a slot jet during incipient and nucleate boiling

The heat transfer due to a submerged slot jet of water impinging on a circular cylinder in cross flow was experimentally studied in the region of incipient and fully developed nucleate boiling. The diameter of the cylinder and the width of the jet are the same-namely, 3 mm. The heat flux ranged from 4 X 10 4 to 1.5 X 10 6 W/m 2 . The effects of the jet velocity and of the fluid subcooling were investigated in the range 0.37 ≤ υ ≤ 1.11 m/s and 0 ≤ (ΔT) sub ≤ 15 K, both in upward and downward flow. The results showed that the influence of these parameters is limited to the region of incipient boiling, whereas, in fully developed boiling, all the boiling curves merge into a single one. A photographic study of the bubble flow pattern evidenced a different behavior with respect to cylinders in uniform cross flow.

U-PHOS Project: Development of a Large Diameter Pulsating Heat Pipe Experiment on board REXUS 22

U-PHOS Project aims at analysing and characterising the behaviour of a large diameter Pulsating Heat Pipe (PHP) on board REXUS 22 sounding rocket. A PHP is a passive thermal control device where the heat is efficiently transported by means of the self-sustained oscillatory fluid motion driven by the phase change phenomena. Since, in milli-gravity conditions, buoyancy forces become less intense, the PHP diameter may be increased still maintaining the slug/plug typical flow pattern. Consequently, the PHP heat power capability may be increased too. U-PHOS aims at proving that a large diameter PHP effectively works in milli-g conditions by characterizing its thermal response during a sounding rocket flight. The actual PHP tube is made of aluminum (3 mm inner diameter, filled with FC-72), heated at the evaporator by a compact electrical resistance, cooled at the condenser by a Phase Change Material (PCM) embedded in a metallic foam. The tube wall temperatures are recorded by means of Fibre Bragg Grating (FBG) sensors; the local fluid pressure is acquired by means of a pressure transducer. The present work intends to report the actual status of the project, focusing in particular on the experiment improvements with respect to the previous campaign.

The U-PHOS experience within the ESA student REXUS/BEXUS programme: A real space hands-on opportunity

U-PHOS (Upgraded PHP Only for Space) is a project developed by a team of students from the University of Pisa with the goal to analyze and characterize the behavior of a Pulsating Heat Pipe (PHP), one of the most attractive two phases passive systems for thermal management in space applications. The PHP consists of a sealed serpentine capillary tube filled with a working fluid. The heat is efficiently transported by means of the combined action of phase change and capillary forces, so no extra equipment is required. The project aims at investigating the thermal response of such a device under a milli-gravity condition, in order to assess its effectiveness in space conditions. U-PHOS is one of the selected experiment of the REXUS/BEXUS programme, which allows European university students to carry out scientific and technical experiments on research rockets and balloons, thanks to a bilateral agency agreement between the German Aerospace Centre (DLR) and the Swedish National Space Board (SNSB) in collaboration with ESA. 19 students from the University of Pisa, with different backgrounds, compose the U-PHOS team. Students had the chance to completely design, build and test the experiment, which will flight up to space in March 2017. This paper intends to describe the work done by the students, their organization and how this experience empowered their careers, from both an academic and professional point of view.

Heat Transfer Enhancement by Ultrasound Pressure Waves in Liquid Water: A Cooling Method for New Electronic Components

The objective of this work is to study the increase of the convective heat transfer coefficient in the presence of ultrasound waves and, in particular, to find a relationship between the ultrasonic power and the effective pressure of water. The tests have focused on the heat exchange by natural convection, in single-phase conditions, at atmospheric pressure, between a cylinder, heated by means of the Joule effect, and distilled water, with and without the action of ultrasonic waves. The variables involved in the heat exchange have been varied within the range allowed by the experimental apparatus and the convective heat transfer coefficient has been maximized. The specific thermal flow has been chosen in a range compatible with applications in the field of electronics. For the first time the pressure of water at various ultrasonic power levels has been measured, in the vicinity of the cylindrical surface, thanks to the collaboration with the Naval Experimentation and Support Center (CSSN) of the Italian Navy in La Spezia, Italy.

Heat Transfer Enhancement Using Ultrasonic Waves in Presence of Liquid: A Basic Research for Cooling Electronic

This work collects the experimental results obtained in the Thermal Fluid Dynamics Lab at the Department of Energy, Systems, Land and Constructions Engineering at the University of Pisa, concerning a basic physics research on the influence of ultrasounds in single phase free convection and in subcooled boiling, at atmospheric pressure. The ultrasounds are applied at the set frequency of 40 kHz, with a transducer output changing from 300 to 500W, on a circular horizontal cylinder heated by Joule effect, immersed in distilled water. The tests in single phase free convection, without ultrasonic waves, are validated by means of the classical correlations reported in literature, but they do not produce distinctive augmentation of the heat transfer. The enhancement of the heat transfer coefficient is maximum in subcooled boiling conditions (about 57%). In this regime a detailed investigation was performed to optimize the variables involved, such as the ultrasound generator power, the position of the cylinder and, especially, the subcooling degree. This paper, makes clear systematically the effects of ultrasounds on the heat transfer and shows as they could be very useful as cooling system for the last generation electronic components.

Heat Transfer Enhancement From a Circular Cylinder to Distilled Water by Ultrasonic Waves at Different Subcooling Degrees

The main aim of this work is to investigate experimentally the influence of ultrasonic waves, on the heat transfer enhancement, from a stainless steel circular cylinder to distilled water, in subcooled boiling conditions. This study has carried on for a few years at the Department of Energetics “L.Poggi”. The effect was observed since ’60s: different authors had investigated the cooling effect due to the ultrasonic waves at different heat transfer regimes, especially from a thin platinum wire to water. They had found out that the highest heat transfer coefficient enhancement was been in subcooled boiling conditions. So this paper has the purpose to clarify the physical phenomenon and optimize a large range of variables involved in the mechanism. It reports the experimental results obtained with ultrasound at the frequency of 38 kHz, at two different subcooling degrees, ΔTSUB = 25 and 35°C. The heat fluxes applied on the cylinder, the ultrasonic generator power, Pgen and also the placement of the heater inside the ultrasonic generator tank, are been varied. The ultrasonic waves seem to be very useful for a practical application in the last generation electronic components cooling: they need dissipating huge heat fluxes and avoiding high temperatures (≈150°C), after that they could damage themselves.© 2010 ASME

“Forced convection between a wire and an upward flow slot submerged jet: preliminary results”

Heat transfer from a platinum wire 0.2 mm. in dia., heated by Joule effect, to an impinging upward flow submerged slot jet of distilled water is studied in two–phase conditions. A new experimental apparatus is built for this experimental activity. Different geometrical configurations were investigated in order to find out which of them could maximize the heat transfer coefficient. Its dependence on some parameters as jet velocity, heat flux and distance between exit jet and wire is also examined. In the future the results of this paper will be compared with the previous ones presented in literature, referred to cylinders of one size order bigger than the platinum wire and the same slot, all parameters being equal.Copyright